1. Field of the Invention
This invention relates to an index guided semiconductor laser device.
2. Description of the Prior Art
In recent years, semiconductor laser devices have been widely used in compact disc players, video disc players, laser printers, optical disc files, etc. Semiconductor laser devices are mainly classified into two groups based on the optical waveguide mechanism, one of which is of a gain guided type and the other of which is of an index guided type. Gain guided semiconductor laser devices are inferior in that they produce an unstable transverse mode and bring about an astigmatism to a large extent, which causes difficulties in the attainment of the functions thereof or difficulties in the use thereof. On the other hand, index guided semiconductor laser devices are superior in practical use in that they produce a stable transverse mode and bring about no astigmatism.
Typical examples of index guided semiconductor laser devices are BH (buried hetero) lasers and VSIS (V-channeled substrate inner stripe) lasers. BH lasers are designed to bury the active region in semiconductor materials having a low refractive index, so that an index guided structure can be created based on a difference in the effective refractive index between the active region and the burying region. When the width of the active layer of the active region is set at a small value (about 2 .mu.m or less), BH layers attain laser oscillation in a stable fundamental transverse mode at a low threshold current level. Moreover, astigmatism is not observed. However, BH lasers confine light within the active layer to an extremely large extent. Thus, in order to cut off high-order transverse modes, the width of the active layer must be made small, which causes difficulties in obtaining a wide area for light-emission and in the creation of high output power.
On the contrary, VSIS lasers are designed to form the active region on a substrate having a striped channel, so that an index guided structure can be formed by the effective refractive index difference based on a difference in light absorption between the region inside of the channel and the region outside of the channel. Since the effective refractive index difference depends upon the distance between the active layer and the substrate, when said distance is set to be small so as to attain laser oscillation in a stable fundamental transverse mode, the absorption of light in not only a high-order transverse mode but also a fundamental transverse mode becomes large, which causes difficulties in the creation of high output power. When the distance between the active layer and the substrate is made long so as to reduce light absorption, the fundamental transverse mode cannot be maintained at a stable level in a range in which high output power can be created.
As mentioned above, it is difficult to produce high output power with conventional BH lasers and conventional VSIS lasers cannot attain stable fundamental transverse mode oscillation in the high output power range.